The pyrolysis at 300 °C. of vinylidene chloride monomer adsorbed on Saran charcoal alters the pore structure of the charcoal so that low boiling gases such as nitrogen are adsorbed slowly. The rates of adsorption of nitrogen, argon, and methane have been measured. They were found to vary with pressure and temperature, and from the temperature variation an activation energy may be calculated. A new method of determining this energy is described which involves changing the temperature during only one adsorption experiment.